How to Calculate KCAT and VMAX

By Sean Butner
Enzyme reactions convert a substrate into a new product without changing or expending the enzyme.
tintenbild ii image by martin schmid from

Enzymes increase the speed of reactions by catalyzing the substrate. An enzyme binds with a substrate, changing the substrate into a new product. Throughout an enzyme reaction, the enzyme remains unchanged. The Michaelis-Menten equation describes the rate of conversion at a given substrate concentration and can help calculate the amount of time required to convert a substrate. The equation requires calculating the Vmax, which is the maximum rate of conversion. The maximum rate of conversion can be defined as the product of the catalyst rate constant (Kcat) and the concentration of the enzyme.

Step 1

Obtain rates of conversion for increasing concentrations of a substrate in a constant concentration of enzyme. Divide one by the rate of conversion for each data point to find the inverse of the rate of conversion. Find the inverse of concentration of the substrate as well.

Step 2

Draw a graph with the y-axis representing the inverse of rate of conversion and the x-axis representing the inverse of concentration of substrate. Plot each data point as inverted above. The plot should form a straight line increasing from left to right.

Step 3

Find the point at which the line intersects the y-axis. Divide the one by the y-value of that point. Record the result as the maximum rate of conversion or Vmax.

Step 4

Divide the Vmax by the concentration of the enzyme. Record the result as the catalyst rate constant or Kcat.

About the Author

Sean Butner has been writing news articles, blog entries and feature pieces since 2005. His articles have appeared on the cover of "The Richland Sandstorm" and "The Palimpsest Files." He is completing graduate coursework in accounting through Texas A&M University-Commerce. He currently advises families on their insurance and financial planning needs.